1. Field of the Invention
This invention relates to an electrophotographic photosensitive member having a sensitivity to electromagnetic waves such as light (which herein refers to light in a broad sense and indicates ultraviolet rays, visible rays, infrared rays, X-rays, Y-rays and so forth).
2. Related Background Art
In the field of image formation, photoconductive materials that form light-receiving layers in light-receiving members such as electrophotographic photosensitive members are required to have properties as follows: They are highly sensitive, have a high SN ratio [photo-current (Ip)/dark current (Id)], have absorption spectra suited to spectral characteristics of electromagnetic waves to be applied, have a high response to light, have the desired dark resistance value and are harmless to human bodies when used. In particular, in the case of electrophotographic photosensitive members set in electrophotographic apparatus used as business machines in offices, the harmlessness in their use is an important point.
The Photoconductive materials having good properties in these respects include amorphous silicon, and have attracted notice as light-receiving layers of electrophotographic photosensitive members.
For such light-receiving members, it is common to form photoconductive layers comprised of a-Si, by film-forming processes such as vacuum deposition, sputtering, ion plating, thermal CVD, photo-assisted CVD and plasma-assisted CVD, which layers are formed on conductive supports while heating the supports at 50° C. to 350° C. In particular, their formation by the plasma-assisted CVD is preferable and has been put into practical use; the plasma-assisted CVD, that is, a process in which source gases are decomposed by high-frequency or microwave glow discharging to form a-Si deposited films on the support.
For example, Japanese Patent Application Laid-Open No. 57-115556 discloses a technique in which a surface barrier layer formed of a non-photoconductive amorphous material containing silicon atoms and carbon atoms is provided on a photoconductive layer formed of an amorphous material composed chiefly of silicon atoms, in order to achieve improvements in electrical, optical and photoconductive properties such as dark resistance, photosensitivity and response to light and service environmental properties such as moisture resistance and also in stability with time, of a photoconductive member having a photoconductive layer constituted of an a-Si deposited film.
Japanese Patent Application Laid-Open No. 6-83090 (corresponding to U.S. Pat. No. 5,464,721) also discloses a contact-charging, negative-charging electrophotographic photosensitive member provided on a photoconductive layer with a charge-trapping layer and a charge injection blocking layer which are formed of a doped a-Si, in order to perform sufficient charging even at the time of high humidity.
Japanese Patent Application Laid-Open No. 6-242623 (corresponding to U.S. Pat. No. 5,556,729) still also discloses a technique in which a hole-capturing layer composed chiefly of amorphous silicon and also containing less than 50 ppm of boron or not containing any element which governs the conductivity is provided between a photoconductive layer and a surface protective layer of a negative-charging electrophotographic photosensitive member to achieve superior electrophotographic performance.
The above techniques have brought improvements in electrical, optical and photoconductive characteristics and service environmental properties, and, with such improvements, have brought an improvement in image quality.
Moreover, in recent years, there are strong desires for improvements in film quality and processability, and measures therefor are studied in variety.
In particular, a plasma-assisted process making use of high-frequency power is widely used because of its various advantages such that it has a high discharge stability and can be used to form insulating materials such as oxide films and nitride films.
In recent years, plasma-assisted CVD carried out at a high frequency of 50 MHz or above using a parallel flat plate type plasma-assisted CVD apparatus, as reported in Plasma Chemistry and Plasma Processing, Vol. 7, No. 3 (1987), pp.267 to 273, has attracted notice, which shows a possibility of improving the deposition rate without a lowering of the performance of deposited films by making the discharge frequency higher than 13.56 MHz conventionally used. Making the discharge frequency higher in this way is also reported in respect of sputtering, and is widely studied in recent years.
When a-Si photosensitive members produced by these processes are applied to electrophotographic apparatus, as charging and charge-eliminating means, corona assemblies (Corotron, Scorotron) are used which have a wire electrode (a metal wire such as a tungsten wire of 50 to 100 μm diameter, coated with gold) and a shielding plate as chief constituent members in almost all cases. More specifically, corona electric currents generated by applying a high voltage (about 4 to 8 kV) to the wire electrode of a corona assembly are made to act on the surface of the photosensitive member to charge its surface and eliminate charges therefrom. Corona assemblies are superior in uniform charging and charge elimination.
However, corona discharge is accompanied with generation of ozone (O3), which oxidizes nitrogen in the air to produce nitrogen oxides (NOx). The nitrogen oxides thus produced further react with water in air to produce nitric acid and so forth. Then, corona discharge products such as nitrogen oxides and nitric acid may adhere to and deposit on the photosensitive member and its surrounding machinery to contaminate their surfaces.
Such corona discharge products have so strong moisture absorption that the photosensitive member surface having adsorbed them comes to have a low resistance because of the moisture absorption of the corona discharge products having adhered thereto, so that the ability of charge retention may substantially lower on the whole or in part to cause image defects such as faint images and smeared images (the electric charges on the surface of the photosensitive member leak in the surface direction to cause deformation, or no formation, of patterns of electrostatic latent images).
Corona discharge products having adhered to the inner surface of the shielding plate of the corona assembly also evaporate and become liberated not only while the electrophotographic apparatus is driven but also while the apparatus is in pause, e.g., at night. Such products adhere to the surface of the photosensitive member at its part corresponding to the discharge opening of the charging assembly to cause further moisture absorption and make the surface of the a-Si photosensitive member have a low resistance. Hence, the first copy initially put out when the apparatus is again driven after a pause of the apparatus, or copies on several sheets subsequent thereto, tend(s) to have smeared images occurring at the area corresponding to the discharge opening that has stood while the apparatus had been in pause. This tends to occur especially when the corona assembly is an AC corona assembly.
Accordingly, a method is available in which a heater for heating the the a-Si photosensitive member is built in the a-Si photosensitive member or warm air is blown on the a-Si photosensitive member by means of a warm-air blower to heat the surface of the a-Si photosensitive member (to 30 to 50° C.) to lower relative humidity. This method is a measure by which the corona discharge products and water content having adhered to the surface of the a-Si photosensitive member are made to volatilize to keep its surface from coming to have low resistance substantially, and has been put into practical use.
As another technique, as disclosed in Japanese Patent Application Laid-Open No. 61-289354, a method is also available in which, in order to keep the initial-stage smeared images from occurring, the surface of the a-Si photosensitive member is made to have an improved water repellency to keep the corona discharge products and water content from adhering to the surface of the a-Si photosensitive member, and has been put into practical use.
As a means for removing the corona discharge products and water content having adhered to the surface of the a-Si photosensitive member, also employed are a cleaning system making use of a magnet roller having a high cleaning ability and a cleaning system making use of a blade.
However, with regard to such a blade type cleaning system, its cleaning performance depends greatly on the slipperiness of the surface of the a-Si photosensitive member. Especially in the field of high-speed copying machines or in the field of laser beam printers or the like, copies or prints are made on a large number of sheets over a long period of time with higher frequencies than usual copying machines. If any a-Si photosensitive members with poor surface slipperiness are used in such copying machines or printers, they have so high frictional resistance to a cleaning blade that the blade can not withstand their long-term service to deteriorate greatly on and on, so that the residual developer (toner) may slip through to cause faulty cleaning in black stripes.
On the other hand, in a-Si photosensitive members with good surface slipperiness, their surface layers may have a tendency to wear greatly to shorten the lifetime of the a-Si photosensitive member.
Such high frictional resistance of the surface of the a-Si photosensitive member may also increase frictional heat between the surface of the a-Si photosensitive member and the cleaning blade to cause a phenomenon of melt adhesion that any residual developer involved in heat fixing adheres toughly to the surface of the a-Si photosensitive member because of this frictional heat. This phenomenon of melt adhesion is slight enough not to affect images at the initial stage, but minute deposits caused by melt adhesion serve as nuclei from which they grow gradually with repeated service to become causes of image defects such as black dots, white dots, black-line blank areas and white-line blank areas appearing on images.
Accordingly, it has become important to prevent the smeared images and the faulty cleaning and also to keep the surface of the a-Si photosensitive member from wearing.
The conventional photosensitive members constituted of a-Si materials have individually been improved in properties in respect of electrical, optical and photoconductive properties such as dark resistance, photosensitivity and response to light as well as service environmental properties and running performance. However, it is actual circumstances that there is room for further improvements in order to achieve overall improvements in properties.
In recent years, with spread of computers and advance of networks in offices, electrophotographic apparatus are not only used as conventional analog copying machines but also now sought to be made digital so that they can play a role as facsimile machines or printers. Moreover, digital full-color copying machines for full-color reproducing digitized information are demanded. Thus, an electrophotographic photosensitive member that can meet such demands are earnestly desired.
In digital full-color copying machines, a negative toner having a wide range of material selection as a toner and an image exposure method (method in which images areas are exposed to light), having a high latent-image controllability and readily achievable of high image quality, are considered to be most common combination for charging, development and so forth. In such a case, it is necessary for the photosensitive member to be charged with negative electric charges. Negative-charging a-Si photosensitive members may preferably be provided with an upper-part charge injection blocking layer in order to block as far as possible the injection of negative electric charges from the surface. How this upper-part charge injection blocking layer be improved holds the key to improvement in properties and characteristics. In particular, to meet a demand for digital full-color copying machines, it has become necessary to make overall improvements in photosensitive member performances. For example, as one of process conditions, a plurality of developing assemblies are provided around one electrophotographic photosensitive member, or a large-size developing means is used. Hence, the machine may have construction where the distance from a charging assembly to a developing assembly tends to be large. Accordingly, the charge potential must be made higher than ever in order to compensate any lowering of potential coming from the charging assembly to the developing assembly, and the upper-part charge injection blocking layer has become important more and more.
In addition, the trend toward higher image quality of the digital full-color copying machines have raised the level of a demand for image quality, and has reached a situation that image defects of an extent that has been tolerated in conventional-type apparatus must be questioned. For example, depending on conditions for producing negative-charging a-Si photosensitive members in which the upper-part charge injection blocking layer is formed, image defects called “pressure mar marks” may appear when a high load is applied to a minute area of the surface of an photosensitive member having been produced. This is a phenomenon that, although any mars (pressure mars) are not seen in appearance at all on a photosensitive member surface when the surface of the photosensitive member is scratched with a diamond needle of 0.8 mm in diameter as tip diameter under application of a load, the ability to retain-dark potential lowers greatly at the part thus scratched, to cause image defects on images.
Such pressure mar marks tend to be conspicuous especially on halftone images. Also, slight pressure mars may varnish upon heating the photosensitive member for about 1 hour at 200° C. to 240° C. However, if the pressure mars have formed in the market, such a measure is impossible to take, and also it is difficult to predict the occurrence of pressure mars.
In addition, as stated previously, in the case when the surface of the a-Si photosensitive member has a high frictional resistance, such high frictional resistance may increase frictional heat between the surface of the a-Si photosensitive member and the cleaning blade to cause the phenomenon of melt adhesion that any residual developer involved in heat fixing adheres toughly to the surface of the a-Si photosensitive member because of this frictional heat. This phenomenon of melt adhesion is slight enough not to affect images at the initial stage, but minute deposits caused by melt adhesion serve as nuclei from which they grow gradually with repeated service to become causes of image defects such as black dots, white dots, black-line blank areas and white-line blank areas appearing on images.
Accordingly, it has become important to prevent the smeared images and the faulty cleaning and also to keep the surface of the a-Si photosensitive member from wearing.
Moreover, problems as discussed below have newly come to pass.                Developers (color toners) used in digital full-color copying machines are non-magnetic toners not containing any magnetic material, where any cleaning system using a magnet roller can not be used. Hence, it has come necessary to effectively bring out the cleaning ability the cleaning blade has.        Image defects in black spots or white spots, i.e., image defects called “dots” are put to severer standards year by year, and images are treated as being poor in some cases even when only few dots are present in an A3-size sheet, depending on their size. Moreover, where electrophotographic photosensitive members are set in color copying machines which are digital copying machines, the standards have come much severer, and images are treated as being poor in some cases even when only one dot is present in an A3-size sheet. Accordingly, an a-Si photosensitive member is desired which may much less cause image defects.        
Thus, the upper-part charge injection blocking layer formed in the conventional negative-charging a-Si photosensitive members is an important part which influences electrophotographic performances, and is demanded to be more improved in regard to the matching with electrophotographic apparatus.